Signals containing noise typically require filtering before they can be used to generate a visual display or to control a physical operation. Sometimes these signals are differentiated to obtain a rate signal. Differentiation typically causes the noise present in the original signal to be accentuated in the rate signal. For example, a noisy signal may generate visual displays which fluctuate excessively or control signals which do not result in a smoothly controlled operation. Even with a noise-free digital signal, step changes in the signal can result in large fluctuations in the rate signal. When the step changes in a digitally encoded signal are comparable in size or larger than the minimum step change of the display or of the motion control, and these changes are unfiltered, the response can be jerky or erratic.
Some modern commercial aircraft utilize digital numerical displays of flight parameters such as cabin altitude and cabin altitude rate. If these signals are not properly filtered before driving a display, a small change in altitude may be displayed initially as a very large change. For example, an unfiltered noisy signal which represents a gradual change in cabin altitude of one hundred feet per minute may initially generate a displayed change in cabin altitude rate of several thousand feet per minute.
These parameters may change slowly or rapidly depending upon the flight of the aircraft. When, for example, cabin altitude is changing at a slow rate, it is desirable to use a filter which has a longer time constant to average the signal over several steps of the input change, thereby achieving a smoothly changing response to the input. However, when cabin altitude is changing at a faster rate, it is desirable to use a filter which has a shorter time constant to permit faster response. When the input is changing faster, the digital step changes occur more frequently so several step changes can be averaged by a shorter time constant.
A prior solution to this problem was to process the input signal using a filter having a fixed time constant. However, to generate a smoothly changing output in response to slowly changing signals, it was necessary to provide a time constant which was significantly longer than the time between changes in the step value. This prevented quick response to inputs having large step changes or inputs which had rapidly changing values.
A number of conventional filters have been disclosed. For example, in U.S. Pat. No. 4,654,811 by Jakubzick there is disclosed an adaptive digital filter having a time constant which is set to a high value when the incoming data changes at a slow rate, and which is set to a low value when the incoming data changes at a faster rate.